Optical scanning apparatus with dynamic scan path control

Abstract

The system includes a primary scanner which produces a scanning optical beam, and a photoresponsive error sensor which measures over the scan path the successive differences between the actual position of the scanning beam and the desired position of the scanning beam and produces error signals. The secondary scanner is connected to the error sensor for response to the error signals to dynamically correct the position of the scanning beam during the course of the scan. The primary scanner provides a main optical scanning beam and an auxiliary optical scanning beam traversing substantially the same optical path. The error sensor includes a graticule mask having a substantially uniform optical density along the desired scan path and a graded optical density transverse to the desired scan path of the auxiliary scanning beam.

We claim: 
     
       1. A scanning optical beam system comprising a primary scanning means which produces a scanning optical beam having a scan path close to the desired scan path, a photoresponsive error sensing means which measures over substantially the entire scan path the successive differences transverse to the scan direction between the actual position of the scanning beam and the desired position of the scanning beam and produces error signals, and a secondary scanning means connected to said error sensing means for response to said error signals and operable to dynamically correct the position of the scanning beam transverse to the scan direction during the course of the scan so as to reduce the measured errors. 
     
     
       2. A system as claimed in claim 1 wherein said primary scanning means includes means for providing a main optical scanning beam and means for providing an auxiliary optical scanning beam traversing substantially the same optical path as said main scanning beam but including means for separation of at least one end of the auxiliary beam from the main beam, and said error sensing means including means arranged in the separated portion of the path of said auxiliary beam to provide for detection of the successive differences between the actual position of the scanning beam and the desired position of the scanning beam in terms of the position of said auxiliary beam. 
     
     
       3. A system as claimed in claim 2 wherein said auxiliary beam defines an original beam path and wherein there is provided within said separated portion of said auxiliary beam a retro-reflector graticule having a substantially uniform average reflectivity along the desired scan path and a graded average optical reflectivity transverse to the desired scan path, said graticule retro-reflector being arranged to reflect the radiation of the auxiliary beam in a reverse direction path back along the original path of the auxiliary beam, a beam splitter arranged in the reverse direction path of said auxiliary beam and operable to intercept and split off at least a portion of the auxiliary beam reflected by said retro-reflector graticule, and said photoresponsive error sensing means being positioned to receive the radiation from said beam splitter as modified by said retro-reflector graticule, said error sensing means being connected to said secondary scanning means to dynamically correct the positions of the main scanning beam. 
     
     
       4. A system as claimed in claim 2 wherein said means for providing an auxiliary optical scanning beam includes an auxiliary scanning beam source, and wherein there is provided within said separated portion of said auxiliary beam a graticule mask having a substantially uniform average optical density along the desired scan path and a graded average optical density transverse to the desired scan path, said photoresponsive error sensing means being positioned beyond said mask on the side opposite to the auxiliary scanning beam source and arranged to receive illumination signals from said auxiliary scanning beam as modified by said mask, said error sensing means being connected to said secondary scanning means to dynamically correct the positions of the main scanning beam. 
     
     
       5. A system as claimed in claim 4 wherein said desired scan path is substantially straight. 
     
     
       6. A system as claimed in claim 4 wherein said desired scan path is curvilinear. 
     
     
       7. A system as claimed in claim 4 wherein said secondary scanning means is operable to dynamically correct the positions of the auxiliary scanning beam as well as the main scanning beam to thereby provide closed loop servo positioning of said scanning beams. 
     
     
       8. A system as claimed in claim 4 wherein the graded optical density of said graticule mask is provided by discrete uniformly shaped and uniformly spaced mask elements distributed along the desired scan path, said elements having wider dimensions along the side of said mask which is to have the greater optical density, and narrower dimensions on the side of said optical mask which is to have the lesser optical density. 
     
     
       9. A system as claimed in claim 8 wherein said mask elements form a pattern resembling the toothed edge of a saw blade. 
     
     
       10. A system as claimed in claim 8 wherein said mask elements comprise elongated substantially parallel figures arranged transverse to the desired beam scan direction and in which said elements are wide on the side of said graticule providing greater optical density and narrow on the side providing lesser optical density. 
     
     
       11. A system as claimed in claim 10 which includes means for transferring information through said scanning beam and means for synchronizing the transfer of information through said scanning beam with the beam scan, said synchronizing means being connected to receive signals from said error sensing means, said synchronizing means being operable in response to the successive interruptions of the signals from said error sensing means arising from the scan of the auxiliary beam across said parallel mask elements to synchronize said information transfer means. 
     
     
       12. A system as claimed in claim 10 in which the scans of said main beam and of said auxiliary beam are intended to be unidirectional, and wherein said mask elements each include a uniformly substantially straight and substantially mutually parallel leading edge which is first intercepted by the auxiliary scanning beam, and wherein the wider dimension on the higher optical density side of each mask element is positioned entirely at the trailing edge which is last scanned by the scanning beam. 
     
     
       13. A system as claimed in claim 12 wherein said mask elements each consist of a narrow line on the low optical density side and a wide line on the high optical density side with a sloped transition profile from the narrow line to the wide line near the center-line of said graticule mask. 
     
     
       14. A system as claimed in claim 4 wherein said primary scanning means includes a laser as the source of illumination. 
     
     
       15. A system as claimed in claim 14 wherein said laser is a helium neon laser. 
     
     
       16. A system as claimed in claim 14 wherein said laser is a gallium arsenside laser. 
     
     
       17. A system as claimed in claim 14 wherein said primary scanning means includes a physically movable mirror. 
     
     
       18. A system as claimed in claim 17 wherein said movable mirror is a rotating polygonal mirror. 
     
     
       19. A system as claimed in claim 14 wherein an auxiliary optical reflector is provided for deflecting said auxiliary scanning beam with respect to said main scanning beam and wherein said auxiliary reflector is associated with said means for separation of said one end of the auxiliary beam from the main beam, said auxiliary reflector being tilted about an axis substantially parallel to the path of scan and perpendicular to the direction of said main scanning beam. 
     
     
       20. A system as claimed in claim 19 including common imaging optical elements for said main beam and said auxiliary beam which are arranged to focus the image of said main beam at a plane corresponding to the surface to be scanned, said graticule being arranged in a conjugate image plane within the separated end of said auxiliary beam. 
     
     
       21. A system as claimed in claim 19 wherein said auxiliary reflector is combined with said means for separation of said one end of said auxiliary beam from said main beam and wherein said auxiliary reflector comprises a beam splitter which reflects part of the beam energy and transmits the rest. 
     
     
       22. A system as claimed in claim 14 including a rotating drum for carrying the scanned image. 
     
     
       23. A system as claimed in claim 22 which is combined in an optical printer. 
     
     
       24. A system as claimed in claim 4 wherein said means of separation of said auxiliary beam from said main beam includes separate sources of illumination for said main beam and said auxiliary beam. 
     
     
       25. A system as claimed in claim 24 wherein said separate illumination sources are lasers of different wave lengths and wherein said means for separation of said main beam and said auxiliary beam includes a wave length selective filter operable to pass one of the beams and to reflect the other one of the beams. 
     
     
       26. A system as claimed in claim 24 wherein said separate light sources comprise two lasers arranged to provide separate main and auxiliary scanning beams traversing the same optical system but in physically separated substantially parallel beams. 
     
     
       27. A system as claimed in claim 26 wherein said two lasers for providing said separate beams are combined in a single dual laser structure. 
     
     
       28. A system as claimed in claim 24 wherein said lasers are gallium arsenide lasers. 
     
     
       29. A system as claimed in claim 24 wherein said light source for the main beam comprises a laser and wherein said error sensing means comprises a photoresponsive pickup device positioned closely adjacent to said laser to receive optical signals from an auxiliary optical scanning beam which traverses substantially the same optical path as the main scanning beam but in a reverse direction from the graticule through the optical path to said error sensing means, said light source for said auxiliary beam being positioned in said optical path on the side of said graticule away from said error sensing means and being operable to illuminate the entire graticule, the optical elements of said optical path being operable to carry an image of the entire graticule back to said error sensing means, and said scanning means being operable to scan the graticule image across said error sensing means, the optical paths of said main and auxiliary beams being closely adjacent to one another and separated at each end of the respective optical paths. 
     
     
       30. A system as claimed in claim 4 including a modulating means for modulating said main beam to provide for optical printing during scan, said means for separation of said auxiliary beam from said main beam being combined in said modulating means, said modulating means comprising an acousto-optic modulator which is arranged in the optical path of the main beam and which is operable during modulation to deflect a part of said main beam to a desired scan position to provide for optical printing at that position, said acousto-optical modulator being operable to pass at least part of the main beam energy continuously in an unmodulated and undeflected beam which comprises the auxiliary beam, and means for deflecting the auxiliary beam away from the surface being scanned to said graticule. 
     
     
       31. A system as claimed in claim 30 including a helium neon laser light source for providing both said main and auxiliary beams. 
     
     
       32. A system as claimed in claim 10 wherein said mask elements are shaped to provide a non-uniform rate of change in width when measured from one side of the graticule to the other, with the greatest rate of change occurring in the central portion across the desired scan path to enhance the sensitivity of the system to changes in transverse positions.